A conventionally known fuel injection control apparatus of a so-called twin needle type adjusts the backside pressures of outer and inner needle valves, which are coaxially accommodated within a valve body, so as to adjustment the lifts of the outer and inner needle valves, to thereby control the injection of fuel (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2005-320904).
FIG. 7 shows an example of a fuel injection control apparatus of this type. A fuel injection control apparatus 10 shown in FIG. 7 includes a fuel pump 20, a common rail 30, injectors 40, an ECU 50 for controlling the fuel pump 20 and the injectors 40, and a fuel tank T.
The fuel pump 20 sucks fuel stored in the fuel tank T and discharges the fuel. The fuel discharged from the fuel pump 20 and having high pressure (rail pressure Pcr) is supplied to the common rail 30. The fuel having the rail pressure Pcr is supplied to the injectors 40 from the common rail 30 through a fuel supply channel C1, which will be described later. Each of the injectors 40 injects the fuel into a combustion chamber (not shown) of an internal combustion engine (particularly, a diesel engine).
The injector 40 has a body 41. The body 41 has first nozzle holes (first nozzle hole group) 41a formed at its tip portion, which faces the combustion chamber of the internal combustion engine, and second nozzle holes (second nozzle hole group) 41b located toward its tip (downward in FIG. 7) with respect to the first nozzle holes 41a. A tubular outer needle valve 42 is slidably accommodated in a predetermined space of the body 41. A tip portion (a lower portion in FIG. 7) of the outer needle valve 42 opens and closes the first nozzle holes 41a. A rod-like inner needle valve 43 is slidably accommodated in the outer needle valve 42. A tip portion (a lower portion in FIG. 7) of the inner needle valve 43 opens and closes the second nozzle holes 41b. 
A cylindrical piece 44 independent of the body 41 is disposed in the predetermined space of the body 41 and is unitarily fixed to the body 41. A lower end portion of the inner circumferential surface of the piece 44 is fitted to an upper end portion of the outer circumferential surface of the outer needle valve 42. Thus, the predetermined space of the body 41 is divided into a nozzle chamber R1 and a control chamber R2.
The nozzle chamber R1 is provided on the tip side of the outer and inner needle valves 42 and 43. The pressure (rail pressure Pcr) of fuel in the nozzle chamber R1 applies force to the outer and inner needle valves 42 and 43 from the tip side in a valve opening direction. In a state in which the outer and inner needle valves 42 and 43 are opened, the fuel in the nozzle chamber R1 is injected into the combustion chamber through the first and second nozzle holes 41a and 41b. 
The control chamber R2 is provided on a back side (upper side in FIG. 7) of the outer and inner needle valves 42 and 43. The pressure (control pressure Pc) of fuel in the control chamber R2 applies force to the outer and inner needle valves 42 and 43 from the back side in a valve closing direction.
The apparatus shown in FIG. 7 has the fuel supply channel C1, a fuel inflow channel C2, and a fuel drain channel C3. The fuel supply channel C1 connects the common rail 30, which stores fuel having the rail pressure Pcr, and the nozzle chamber R1. The fuel inflow channel C2 connects the control chamber R2 and the fuel supply channel C1, and the fuel drain channel C3 connects the control chamber R2 and the fuel tank T. An orifice Z1 is installed in the fuel inflow channel C2 and the fuel drain channel C3.
A 2-position 3-port control valve 45 is installed in the fuel inflow channel C2 and the fuel drain channel C3. The control valve 45 functions such that, when communication is established in the fuel inflow channel C2, the fuel drain channel C3 is shut off (first position as shown in FIG. 7) and such that, when the fuel inflow channel C2 is shut off, communication is established in the fuel drain channel C3 (second position). Hereinafter, the fuel injection control apparatus of the twin needle type shown in FIG. 7 may be called “the first conventional apparatus.” The lifts of the outer and inner needle valves 42 and 43 mean the distances of upward movement (rising distances) of the outer and inner needle valves 42 and 43 from the state shown in FIG. 7.
Next, referring to FIG. 8, an example operation of the above-mentioned first conventional apparatus will be described. Notably, when the outer and inner needle valves 42 and 43 are closed (as shown in FIG. 7; lift=0), a gap δL between an upper end surface 42a (back surface) of the outer needle valve 42 and a lower surface 43a of a flange portion of the inner needle valve 43 is assumed to be a value L1.
When the closed outer and inner needle valves 42 and 43 are to be opened (when a valve closed state is to be changed to a valve opened state (lift>0)), the operational position of the control valve 45 is changed from the above-mentioned first position to the above-mentioned second position (see time tA). By this positional change, the fuel begins to be drained from the control chamber R2 through the fuel drain channel C3. As a result, at and after time tA, the control pressure Pc lowers from the rail pressure Pcr.
In the first conventional apparatus, the outer needle valve 42 is lower than the inner needle valve 43 in the ratio of a control pressure Pc receiving area on the back side to a rail pressure Pcr receiving area on the tip side. Accordingly, an “outer needle valve opening pressure P1” (a control pressure Pc at the time of transfer of the outer needle valve 42 from a closed state to an opened state) is higher than an “inner needle valve opening pressure P2 (a control pressure Pc at the time of transfer of the inner needle valve 43 from the closed state to the opened state).
Thus, when the control pressure Pc which is lowering from the rail pressure Pcr reaches the outer needle valve opening pressure P1, only the outer needle valve 42 opens (moves upward in FIG. 7). As a result, fuel injection is started and performed only through the first nozzle holes (first nozzle hole group) 41a (see time tB). Hereinafter, the time when the outer needle valve 42 opens may be called “the outer valve opening time.”
When the outer needle valve 42 opens, the fuel having the rail pressure Pcr enters between the outer needle valve 42 and an outer needle valve seat portion 41c. For this reason, only immediately after the outer valve opening time, the outer needle valve 42 rises at a speed corresponding to the differential pressure between the rail pressure Pcr and the control pressure Pc. Subsequently, the outer needle valve 42 rises at a speed corresponding to the flow rate of fuel passing through the orifice Z1 (outflow rate Qout). Also, this speed of the outer needle valve 42 depends on the rate of change of the control pressure Pc.
The upper end surface 42a of the outer needle valve 42 which moves upward as mentioned above comes into contact with the lower surface 43a of the flange portion of the inner needle valve 43 (i.e., the gap δL becomes 0; see time tC). Subsequently, the outer and inner needle valves 42 and 43 can rise only unitarily. Hereinafter, a unitary body of the outer and inner needle valves 42 and 43 may be called “the unitary needle valve.” The time when the upper end surface 42a of the outer needle valve 42 comes into contact with the lower surface 43a of the flange portion of the inner needle valve 43 may be called “the needle valve contact time.”
When the lowering control pressure Pc reaches the inner needle valve opening pressure P2, the inner needle valve 43 also opens (moves upward in FIG. 7). As a result, fuel injection is started and performed also through the second nozzle holes (second nozzle hole group) 41b (see time tD). Hereinafter, the time when the inner needle valve 43 opens may be called “the inner valve opening time.”
Similar to the outer needle valve 42, in this unitary needle valve (inner needle valve 43), when the inner needle valve 43 opens, the fuel having the rail pressure Pcr enters between the inner needle valve 43 and an inner needle valve seat portion 41d. For this reason, only immediately after the inner valve opening time, the inner needle valve 43 rises at a speed corresponding to the differential pressure between the rail pressure Pcr and the control pressure Pc. Subsequently, the inner needle valve 43 rises at a speed corresponding to the outflow rate Qout. Also, this speed of the inner needle valve 43 depends on the rate of change of the control pressure Pc.
When the opened outer and inner needle valves 42 and 43 are to be closed (when the valve opened state is to be changed to the valve closed state), the operational position of the control valve 45 is changed from the second position to the first position (see time tE). By this positional change, the drainage of fuel from the control chamber R2 through the fuel drain channel C3 is halted, and the inflow of fuel into the control chamber R2 through the fuel inflow channel C2 is started. As a result, at and after time tE, the control pressure Pc rises toward the rail pressure Pcr.
At and after time tF, which is slightly after time tE, the unitary needle valve lowers (moves downward in FIG. 7), and, first, the inner needle valve 43 closes (see time tG). Accordingly, fuel injection through the second nozzle holes (second nozzle hole group) 41b ends. Subsequently, the outer needle valve 42 lowers independent of the inner needle valve 43 and then closes (see time tH). Accordingly, fuel injection through the first nozzle holes (first nozzle hole group) 41a also ends. Hereinafter, the times when the outer and inner needle valves 42 and 43 close may be called “the outer valve closing time” and “the inner valve closing time,” respectively. In this manner, the control valve 45 is controlled so as to control the control pressure Pc, whereby the lifts of the outer and inner needle valves 42 and 43 are adjusted, thereby controlling fuel injection.